Optical fiber cable ready for mid-span access and method of mid-span access thereof

ABSTRACT

An optical fiber cable is comprised of: a slotted core ( 7 ) elongated along an axis of the optical fiber cable, the slotted core including a slot ( 11 ) running in parallel with the axis and a groove ( 5 ) accessible through the slot; one or more optical fibers ( 3 ) placed in the groove; a sheath ( 9 ) enclosing the slotted core and the optical fibers; a bonding portion ( 15 ) where the slotted core is bonded with the sheath; and two or more strength members ( 17 ) embedded in the slotted core, the strength member running in parallel with the axis, and being aligned on a plane including the axis.

TECHNICAL FIELD

Apparatuses consistent with the present invention relate to optical fiber cables enclosing fibers, in which enclosed fibers are easily accessible but prevented from damage, and a method of mid-span access of the optical fiber cables.

BACKGROUND ART

An optical fiber cable in some cases includes plural fibers for the purpose of increasing the capacity or the number of devices linking via the cable. These fibers may be enclosed with a slotted core and the slotted core along with the fibers may be further enclosed with a sheath.

After being laid, some optical fiber cables are often subject to a work named “mid-span access” to make the enclosed optical fibers branch off. In the mid-span access work, the sheath and the core are cut and split to enable access to one or more of the enclosed fibers. As the fibers exposed to the exterior are susceptible to damage, any proper measure for protection may be required.

Japanese Patent Unexamined Application Publications Nos. S62-291608, S63-5313, H06-50009 and H08-211261 disclose related arts of optical fiber cables.

DISCLOSURE OF INVENTION Technical Problem

Some circumstances cause damage to properties of the optical fibers. For example, as the slotted core is likely to move relative to the sheath, projection of the slotted core out of one end of the sheath may occur. The projection will lead to damage to the optical fibers at the projecting part. Further, curving or meandering may generate compression or tensile stress on the optical fibers, which causes increase in transmission loss. Certain embodiments of the present invention provide optical fiber cables enclosing fibers, in which enclosed fibers are easily accessible but prevented from damage.

Technical Solution

An optical fiber cable according to an aspect of the present invention has an axis. The optical fiber cable is comprised of: a slotted core elongated along the axis, the slotted core including a slot running in parallel with the axis and a groove accessible through the slot; one or more optical fibers placed in the groove; a sheath enclosing the slotted core and the optical fibers; a bonding portion where the slotted core is bonded with the sheath; and two or more strength members embedded in the slotted core, the strength member running in parallel with the axis and being aligned on a plane including the axis.

According to another aspect of the present invention, a method of mid-span access of the optical fiber cable is comprised of separating a part of a sheath from the optical fiber to expose the slot in part; covering the exposed slot in part with the separated part of the sheath; and pulling one or more of the optical fibers out of the slotted core.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross section of an optical fiber cable according to a first embodiment of the present invention.

FIGS. 2(A) through 2(E) are drawings explaining a process of mid-span access.

FIG. 3 is a schematic drawing explaining a draw test method.

FIG. 4 shows a cross section of an optical fiber cable according to a second embodiment of the present invention.

FIG. 5 shows a cross section of an optical fiber cable according to a fourth embodiment of the present invention.

FIG. 6 shows a cross section of an optical fiber cable according to a fifth embodiment of the present invention.

FIG. 7 shows a cross section of an optical fiber cable according to a sixth embodiment of the present invention.

FIG. 8 shows a cross section of an optical fiber cable according to a seventh embodiment of the present invention, which further applies to eighth and ninth embodiments of the present invention.

FIG. 9 shows a cross section of an optical fiber cable according to a tenth embodiment of the present invention.

FIG. 10 shows a longitudinal section of the optical fiber taken along the Y-axis of FIG. 9.

FIG. 11 shows a cross section of an optical fiber cable according to an eleventh embodiment of the present invention.

FIGS. 12(A) through 12(C) are partial sectional views to show variations of markers for indicating where a slotted core is fixed with a sheath.

FIG. 13 shows a cross section of an optical fiber according to an embodiment of the present invention, which is replaceable with that of the first embodiment.

FIG. 14 shows a cross section of an optical fiber according to a fourteenth embodiment of the present invention.

FIGS. 15(A) through 15(D) are drawings explaining a process of mid-span access explained in relation to the optical fiber cable according to the fourteenth embodiment.

FIG. 16 is a cross sectional view of the optical fiber cable for explanation of the process of the mid-span access.

FIG. 17 is a cross sectional view of the optical fiber cable for illustrating a state after splitting.

BEST MODE FOR CARRYING OUT THE INVENTION

Exemplary embodiments of the present invention will be described hereinafter with reference to the appended drawings. While optical fiber cables according to the embodiments are elongated along a central axis C thereof, FIGS. 1, 4-9, 11-13 only show cross sections thereof taken along a plane perpendicular to the central axis. The following descriptions and the appended drawings often refer rectangular coordinates represented by X- and Y-axes on these sectional planes for convenience of explanation. These X- and Y-axes and elements related thereto sometimes represent planes and bodies elongated along the central axis C.

Referring to FIG. 1, an optical fiber cable 1 according to a first embodiment of the present invention is comprised of optical fibers 3, a slotted core 7 having a groove 5 for housing the optical fibers 3, and a sheath 9 enclosing the slotted core 9 along with the optical fibers 3. Needless to say, all the fibers 3, the groove 5, the core 7, the sheath 9 and the slot 11 run in parallel with the central axis C of the optical fiber cable 1.

The slotted core 7 is further comprised of a slot 11 opened linearly along the slotted core 7 for enabling access to the interior of the groove 5. Therefore the slotted core 7 has a C-letter cross sectional shape. The wall of the slotted core 7 gradually becomes thicker toward the side opposite to the slot 11. The groove 5 is eccentric from the outer profile of the slotted core 7. When the center of the slot 11 and the side just opposite to the slot 11 are made aligned on the Y-axis, the eccentricity is also in a direction along the Y-axis.

The sheath 9 preferably consists of any proper resin such as polyethylene. The sheath 9 in comprised of a nonuniform wall which gradually becomes thinner from a thickest wall portion 13 toward a thinnest wall portion 15, both of which are aligned on the Y-axis. Thereby eccentricity in the direction along the Y-axis is given to a hollow defined by the wall relative to the outer profile of the sheath 9. The thickest wall portion 13 covers the slot 11.

As the eccentricity of the groove 5 relative to the slotted core 7 is just reverse to the eccentricity of the hollow of the sheath 9, the groove 5 is resultantly substantially concentric with the central axis C of the optical fiber cable 1. Alternatively, the groove 5 may be slightly eccentric from the central axis C in either direction along the Y-axis.

The slotted core 7 is further comprised of a pair of strength members 17 embedded therein. Both the strength members 17 are aligned on the plane including both the Y-axis and the central axis C of the cable 1. Further, the strength members 17 are in nature opposite to the slot 11 with respect to the central axis C. The strength members 17 may be formed in various shapes such as a line, a strip, an elongated multilateral prism or a column. The number of the strength members 17 is not limited to two and may be three or more. A slight interval is preferably held between the strength members 17.

The strength members 17 are made of any material reinforcing the optical fiber cable 1 against tensile force, such as steel or FRP (Fiber Reinforced Plastic), and in general have greater stiffness than those of the other members. As the strength members 17 having such stiffness are aligned on the plane, when the optical fiber cable 1 is curved, this plane functions as a neutral surface in a meaning of mechanics (a surface along which material is neither compressed nor extended).

In any case, the strength members 17 may be aligned on another plane. Even then, if the optical fibers 3 are disposed around the plane, increase in transmission loss can be suppressed as will be discussed later.

Although the cross sectional shape of the groove 5 is illustrated as a circle in FIG. 1, the shape is not limited thereto and may be an ellipse or any irregular shape instead. Further, the interior of the groove 5 may be either vacant except the optical fibers 3 or filled with any buffer members. In any case, the optical fibers 3 are preferably disposed around the central axis C.

The optical fibers 3 may be any of bare optical fibers, optical fiber cords, and optical fiber ribbons.

An elongate tape 19 preferably made of non-woven fabric or any resin such as PET (PolyEthylene Terephthalate) is attached on the slotted core 7 to cover the slot 11. The elongate tape 19 is not wrapped around the slotted core 7 and leaves a lower part of a surface of the slotted core 7 uncovered. Therefore the sheath 9 may be directly in contact with this lower part of the slotted core 7 while the elongate tape 19 intervenes between the upper part of the slotted core 7 and the sheath 9.

At this uncovered part, the slotted core 7 has a bonding portion 23 where the slotted core 7 is bonded to the sheath 9. The bonding portion 23 longitudinally ranges over the slotted core 7 to form a continuous line or a row of separate portions at intervals. Thermal fusion boding may be applied to bonding at the bonding portion 23. In the present embodiment, a projecting rib 21 projecting from the slotted core 7 is formed in advance of bonding. The projecting rib 21 facilitates thermal fusion bonding with the sheath 9 and, after bonding, becomes the bonding portion 23 fitting in and bonding with a complementary recess of the sheath 9. In any proper case, thermal fusion bonding or any other bonding treatment can be omitted and the projection rib 21 fitting in the recess by itself functions as bond. Preferably the projecting rib 21 does not project out of the sheath 9.

The optical fiber cable 1 may include a rip cord to facilitate splitting the sheath 9.

As already discussed, the plane on which the strength members 17 are aligned, shown as the Y-axis in FIG. 1, functions as a neutral surface in a meaning of mechanics when the optical fiber cable 1 is curved in any directions perpendicular to the plane (namely, in the direction of the X-axis). Moreover the optical fiber cable 1 can substantially freely rotate or twist with very small stress, because the cable 1 is in general subject to very small restriction. Thus, even if one would curve the optical fiber cable 1 in a direction deviated from the X-axis, the optical fiber cable 1 would be slightly reoriented to have itself curved in the X-axis and then the plane including the central axis C still functions as a neutral surface. Further, as the optical fibers 3 are disposed around the central axis C (included in the neutral surface), the optical fibers 3 are substantially neither compressed nor extended. Therefore transmission loss caused by compression or tensile stress can be suppressed in a very low level. It is advantageous in view of suppression of transmission loss particularly when some circumstances force a laid optical fiber cable to curve or meander.

As the sheath 9 is free from a strength member which may be obstructive to splitting or cutting out the sheath 9, the optical fiber cable 1 provides facility for the mid-span access work.

As the sheath 9 has a nonuniform wall in which the thickest wall portion 13 covers the slot 11, mechanical strength in this part is reinforced. This is advantageous in view of prevention of damage to the enclosed optical fibers 3 when external force is applied to the sheath 9, in particular over the slot 11. This effect becomes remarkable when the thickness of the thickest wall portion 13 is 1.5 times or more of the thickness of the thinnest wall portion 15.

Without the bonding portion 23, the slotted core 7 is likely to move in its longitudinal direction because temperature change after laying the optical fiber cable 1 may cause thermal expansion or contraction. Further, some manners of handling of the optical fiber cable 1 may cause rotational displacement of the slotted core 7 relative to the sheath 9. As the sheath 9 and the slotted core 7 are bonded together at the bonding portion 23, the slotted core 7 is prevented from displacement relative to the sheath 9 in both the longitudinal and rotational directions. The bond at the bonding portion 23 effectively prevents projection, retraction and rotational displacement of the slotted core 7. As the bond at the bonding portion 23 prevents such displacement, the optical fiber cable 1 provides prominent facility for handling.

The bond between the slotted core 7 and the sheath 9 is limited in the bonding portion 23. This fact provides facility for the mid-span access work because peeling of the sheath 9 is easily carried out as compared with a case where the core and the sheath are entirely bonded together. In particular, while a cutter is put into the sheath at the beginning of the mid-span access work, the cutter may cut out the projecting rib 21 and therefore simultaneously break the bond between the slotted core 7 and the sheath 9 at the bonding portion 23. Thus workability about the mid-span access work is prominently improved.

Referring to FIGS. 2(A) through 2(E), a process of mid-span access will be described hereinafter. First, a sharp edge of a cutter 25 is put into the sheath 9 and made advance around the circumference thereof to carry out cutting around of the sheath 9. Further the cutter 25 is made advance along the bonding portion 23 to carry out splitting of the sheath 9 and subsequently cutting around is carried out again at another part as shown in FIG. 2(B). As the bonding portion 23 is formed at the projecting rib 21, the bond between the slotted core 7 and the sheath 9 is easily broken in the course of the aforementioned step. Further, while movement of the cutter 25 along the longitudinal direction gives rise to damage to the enclosed optical fibers 3 as the slot 11 runs in this direction, such movement is made at the bonding portion 23 opposite to the slot 11 and therefore the optical fibers 3 are not subject to damage.

Then a part of the sheath 9 is separated from the remainder of the sheath 9 and further a longitudinal slit 27 is formed in this part as shown in FIG. 2(C). This part becomes removable from the slotted core 7 and the optical fibers 3.

After removal of the part of the sheath 9, the slot 11 is exposed as shown in FIG. 2(D) and therefore the optical fibers 3 housed in the groove 5 become accessible through the slot 11. One or more of the optical fibers 3 are pulled out of the slotted core 7 as shown in FIG. 2(E) and then subject to a branching process.

Table 1 demonstrates test results of some examples in regard to a drawing test, projection length of the slotted core at the end of the sheath, workability about the mid-span access work, and transmission loss. The drawing test had been carried out in a manner shown in FIG. 3, in which a slotted core 7 of a test piece 29 is drawn from a sheath 9 having a length of 400 mm in a speed of 100 mm/min in a direction indicated by an arrow therein and a maximum value of force of drawing is measured.

Meanwhile, the force of drawing is preferably 98N or more in view of prevention of displacement of the slotted core relative to the sheath.

The working example 1 is produced in accordance with the present embodiment. Comparative examples 1-5 are different from the present embodiment in structural parameters as summarized in this table.

TABLE 1 MAIN WORKING COMPARATIVE COMPARATIVE COMPARATIVE COMPARATIVE COMPARATIVE FEATURES EXAMPLE 1 EXAMPLE 1 EXAMPLE 2 EXAMPLE 3 EXAMPLE 4 EXAMPLE 5 STRUC- FIXATION PARTIAL OVERALL NONE PRESS ON NONE NONE TURAL BETWEEN THE ADHESION THE SLOT BY PARAM- SLOT CORE THE SHEATH ETERS AND THE SHEATH TAPE OR A TAPE ALONG A TAPE ALONG A TAPE ALONG A TAPE ALONG A SPIRAL A TAPE TOTALLY WRAPPING THE SLOT IN THE SLOT IN THE SLOT THE SLOT WRAPPING ALONG THE PART, PART, IN PART, IN PART, AROUND SLOT, AND A NO WRAPPING NO WRAPPING NO WRAPPING NO WRAPPING THE SLOT YARN WOUND THEREAROUND EVALU- FORCE OF 98 N OR MORE 98 N OR MORE 10 N OR LESS 98 N OR MORE 85 N 20 N ATION DRAWING THE SLOT CORE (FIG. 3) (SHEATH LENGTH: 40 cm) A PROJECTION NONE NONE A PROJECTION A PROJECTION A PROJECTION A PROJECTION FROM THE (1 mm OR LESS) (1 mm OR OF 55 mm OF 5 mm OF ABOUT 5 mm OF 36 mm END OF LESS) IN LENGTH IN LENGTH IN LENGTH IN LENGTH THE SHEATH WORKABILITY EXCELLENT POOR EXCELLENT TOLERABLE TOLERABLE TOLERABLE *2 (EXTRA WORK (EXTRA WORK FOR REMOVAL) FOR REMOVAL) TRANSMISSION 0.21 dB/km 0.21 dB/km 0.21 dB/km 0.45 dB/km 0.23 dB/km 0.21 dB/km LOSS (AT THE WAVELENGTH OF 1.55 μm) *NOTES EXCELLENT: WORKABLE WITHOUT ANY PROBLEMS AND WITH GREATER FACILITY THAN EXISTING CABLES TOLERABLE: WORKABLE WITH CONSIDERABLE LABOR AS COMPARED WITH THAT FOR EXISTING CABLES POOR: TROUBLESOME IN WORK

As being understood from Table 1, the working example 1 in accordance with the present embodiment has satisfactory properties in that the force of drawing is 98N or more, the projection length is 1 mm or less, and the transmission loss is only 0.21 dB/km while workability about the mid-span access work is excellent.

The comparative example 1 is different from the working example 1 in that the slotted core 7 and the sheath 9 are totally bonded together. Workability about the mid-span access work is inferior to that of the working example 1 because it is considerably laborious to peel off the sheath 9 totally bonded with the slotted core 7.

The comparative example 2 is different from the working example 1 in that no bond is formed between the slotted core and the sheath. This structure results in relatively small force of 10N or less required to draw the slotted core out of the sheath and a relatively large projection length of 55 mm of the slotted core out of the sheath. This means that the slotted core is susceptible to displacement relative to the sheath.

The comparative example 3 is different from the working example 1 in that fixation of the slotted core with the sheath depends only on pressure of the sheath onto the slotted core. This structure results in a relatively large projection length of 5 mm of the slotted core out of the sheath. Further, workability about the mid-span access work is inferior to that of the working example 1. Transmission loss increases up to 0.45 dB/km which is considerably larger than 0.21 dB/km of the working example 1.

The comparative example 4 is different from the working example 1 in that no bond is formed between the slotted core and the sheath and a wrapping made of a tape is wound around the slotted core in a spiral shape. As the wrapping serves for friction against displacement of the slotted core, force of drawing is relatively high, 85N. However, projection length of the slotted core out of the sheath reaches about 5 mm. Further, workability about the mid-span access work is inferior to that of the working example 1 as extra work to remove the wrapping is required. Transmission is relatively low, 0.23 dB/km, although this value is slightly larger than that of the working example 1.

The comparative example 4 is different from the working example 1 in that no bond is formed between the slotted core and the sheath and further a wrapping made of a yarn is wound around the slotted core along with the elongate tape along the slot. While the wrapping serves for friction against displacement of the slotted core, force of drawing the slotted core is only 20N and projection length of the slotted core out of the sheath reaches 36 mm. Further, workability about the mid-span access work is inferior to that of the working example 1 as extra work to remove the wrapping is required. Transmission loss is fairly low, 0.21 dB/km.

As being understood from the aforementioned comparisons, the working example 1 in accordance with the present embodiment provides beneficial results as compared with the comparative examples, such as prevention of displacement of the slotted core relative to the sheath, low transmission loss, and excellent workability about the mid-span access work.

The aforementioned embodiment will be modified in various ways. Some of such modifications will be exemplarily described hereinafter. In the following descriptions, differences compared with the aforementioned embodiment will be mainly described and descriptions about elements substantially identical to those of the aforementioned embodiment will be omitted or simplified.

Referring to FIG. 4 which illustrates a second embodiment, the slotted core 7 is in part given roughness in advance of bonding and the rough surface of the slotted core 7 is subject to thermal fusion bonding to form a bonding portion 23 with the sheath 9. The bonding portion 23 is composed of a thermal fusion bonding portion 31 produced by the thermal fusion bonding, where the slotted core 7 and the sheath 9 are fused together and thereby locally form a unitary body.

Alternatively, in a third embodiment, the slotted core 7 is in part heated up to a temperature sufficiently close to, or higher than, that of the sheath 9 in advance of bonding so as to cause softening of the slotted core 7, and then thermal fusion bonding is carried out.

Referring to FIG. 5 which illustrates a fourth embodiment, instead of thermal fusion bonding, a bonding material 33 such as adhesive may be used to form bond between the slotted core 7 and the sheath 9. Therefore the bonding portion 23 is composed of the bonding material 33.

Referring to FIG. 6 which illustrates a fifth embodiment, instead of a projecting rib, a recess 35 receding into the slotted core 7 is formed in advance of bonding and the sheath 9 has a complementary projection. After fitting the projection in the recess 35, thermal fusion bonding is carried out to form a bonding portion 23 therebetween. As with the rib of the first embodiment, the recess 35 may be either a continuous line of a concavity or a row of separate concavities, which longitudinally ranges over the slotted core 7.

Referring to FIG. 7 which illustrates a sixth embodiment, a rip cord 37 having a bonding material such as adhesive is interposed between the slotted core 7 and the sheath 9. The rip cord 37 is preferably disposed on or close to the plane on which the strength members 17 are aligned. By means of the bonding material of the rip cord 37 instead of thermal fusion bonding, the slotted core 7 is bonded with the sheath 9. When the rip cord 37 is drawn, it helps to split the sheath as an ordinary rip cord does, and further simultaneously breaks bond between the slotted core 7 and the sheath 9. As in this way removal of the sheath 9 is further facilitated, one can further easily execute mid-span access work as compared with the first embodiment.

Referring to FIG. 8 which illustrates a seventh embodiment, one or more absorptive yarns 39 may be housed in the groove 5 of the slotted core 7. The absorptive yarns 39 improve quality of being waterproof of the optical fiber cable 1.

Alternatively, an absorptive tape 41 having capacity to absorb water may be applied instead of, or along with, the elongate tape 19 of the first embodiment. The absorptive tape 41 also improves quality of being waterproof.

Further alternatively, both the absorptive yarns 39 and the absorptive tape 41 may be used. This combination of the absorptive yarns 39 and the absorptive tape 41 further improves quality of being waterproof.

Referring to FIGS. 9 and 10 which illustrate a tenth embodiment, anchors 43 are provided in the groove 5 of the slotted core 7. The anchors 43 support one or more of the optical fibers 3 in place. Preferably, the anchors 43 are disposed at intervals in a direction along the central axis C. This structure prevents undesirable force acting on the optical fibers 3 even when the optical fiber cable 1 is deformed. Moreover, the anchors 43 are preferably made of any soft viscous material. Preferably the material is a UV-setting resin having a Young's modulus of 800 MPa or less and a viscosity of 500 cps or more at the normal temperature, whereby preventing undesirable force acting on the optical fibers 3, which may increase transmission loss. Preferably, the intervals of the respective anchors 43 are in a range from 100 mm to 2000 mm, whereby preventing undesirable force acting on the optical fibers 3. Preferably, the support of the optical fibers 3 by the anchors 43 is regulated so that a force required to draw out the supported optical fibers are 5N/10 m or more.

Installation of the anchors 43 is executed in, but not limited to, the following way. The tape 19 is uncoiled and then made to run. An uncured UV-setting resin is intermittently injected onto the running tape 19 substantially at the center thereof. Then the tape 19 along with the uncured UV-setting resin is exposed to UV light so as to cure the UV-setting resin and is subsequently turned around upside down. Thereby the anchors 43 made of the UV-setting resin are disposed at intervals on the lower face of the tape 19. On the other hand, the optical fibers 3 are put in the groove 5 of the slotted core 7 and the slot 11 is made oriented upward. The tape 19 along with the anchors 43 is attached on the slotted core 7 so as to cover the slot 11, whereby the anchors 43 are inserted in the groove 5 to support the optical fibers 3. An extruder may be used to enclose the slotted core 7 with the sheath 9.

Table 2 demonstrates test results of some examples in regard to transmission loss, a drawing test and workability about the mid-span access. The examples 1-8 are in general manufactured in accordance with the aforementioned tenth embodiment and vary in kinds of resin, Young's moduli thereof, and viscosities thereof, as summarized in Table 2.

TABLE 2 TARGET EXAM- EXAM- EXAM- EXAM- EXAM- EXAM- VALUE PLE 1 PLE 2 PLE 3 PLE 4 PLE 5 PLE 6 EXAMPLE 7 EXAMPLE 8 CONDI- FIXATION OF THE — BY A BY A BY A BY A BY A UV BY A UV BY A BY A FILLED TION OPTICAL FIBER UV UV UV UV RESIN RESIN HOT-MELT YARN RESIN RESIN RESIN RESIN ADHESIVE YOUNG'S MODULUS — 500 1000 500 800 1000 600 — 1000 OF THE RESIN (Mpa) VISCOSITY OF — 300 300 500 500 500 700 GREATER 500 THE RESIN (cps) THAN 10000 RE- TRANSMISSION LESS 0.22 0.28 0.21 0.23 0.32 0.20 0.32 0.86 SULTS LOSS (dB/km) THAN 0.25 FORCE OF GREAT- 2.8 4.2 9.8 8.5 12 11.5 4 11 DRAWING THE ER SLOT CORE THAN 5 (N/10 m) WORKABILITY — EX- EX- EX- EX- EXCEL- EXCEL- TOLERABLE TOLERABLE CEL- CEL- CEL- CEL- LENT LENT (EXTRA WORK (EXTRA WORK LENT LENT LENT LENT FOR REMOVAL FOR REMOVAL OF THE OF THE YARN) HOT-MELT ADHESION)

Provided that a target level of performance is set such that a transmission loss is 0.25 dB/km or less, a force required to draw out the slotted core from the sheath is greater than 5, and workability about the mid-span access work is beyond that of existing cables, what meet the target level among the examples are the example 3, 4 and 6, which are commonly comprised of anchors made of the UV-setting resin. Both the example 7 having anchors made of hot-melt adhesive and the example 8 in which yarns filled in the groove fix the optical fibers do not meet the target level.

In more detail, the examples 2, 5 and 8 do not have sufficiently low transmission loss which meets the target level as the Young's moduli of the anchors of these examples reach 1000 MPa. In contrast, the examples 1, 3, 4 and 6 meet the target transmission loss, in which the Young's moduli of the anchors are 800 MPa or less. More specifically, anchor's Young's moduli of 800 MPa or less provide beneficial results in view of suppression of transmission loss.

Further, the examples 1 and 2 in which the viscosities of the anchors are 300 cps do not meet the target force of drawing, whereas the examples 3, 4, 5 and 6 in which the viscosities are 500 cps or more meet the target force of drawing. More specifically, anchor's viscosities of 500 cps or more provide beneficial results in view of prevention of displacement of the slotted core.

Further modification of the above embodiments will occur. Referring to FIG. 11 which illustrate an eleventh embodiment, widths of the slot 11 in a proper range also beneficial results. A plane emanating from the central axis C in contact with an edge in the right of the slot 11 is shown as a line L in FIG. 11 and another plane emanating from the central axis C in contact with another edge in the left of the slot 11 is shown as a line L′. These planes make an angle “theta” as shown in FIG. 11. When the angle theta is larger than 30 degrees, workability about the mid-span access work becomes easy. Further, when the angle theta is less than 90 degrees, the sheath 9 is prevented from falling into the groove 5 and therefore does not have undesirable influence on transmission loss. More specifically, the angles theta in a range from 30 degrees to 90 degrees provide beneficial results.

Further, widths of the tape 19 in a proper range also beneficial results. A plane emanating from the central axis C in contact with an edge in the right of the tape 19 is shown as a line T in FIG. 11 and another plane emanating from the central axis C in contact with another edge in the left of the tape 19 is shown as a line T′. These planes make an angle “gamma” as shown in FIG. 11. Beneficial results provided by angles gamma larger than the angle theta would be needless to say. When the angle gamma is less than four times the angle theta, the slotted core 7 is securely fixed with the sheath 9 as the slotted core 7 and the sheath 9 ensure sufficient contact area. More specifically, the angles gamma in a range from the angle gamma to four times the angle gamma provide beneficial results.

Referring to FIGS. 12(A)-12(C) which illustrate a twelfth embodiment, the optical fiber cable 1 may be further comprised of a marker for indicating a position of the bonding portion 23. The maker may be a projection 45 projecting from the sheath 9, which is just aligned to the bonding portion 23 as shown in FIG. 12(A). Alternatively, the marker may be a colored bar 47 on the sheath 9 as shown in FIG. 12(B). Further alternatively, the marker may be a concave portion 49 as shown in FIG. 12(C). Existence of the marker helps one who would carry out the mid-span access work to find out where to cut.

Referring to FIG. 13 which illustrates a thirteenth embodiment, a pair of rectangular-prism-shape strength members 20 are embedded in the slotted core 7, instead of the columnar strength members 17 of the first embodiment.

Referring to FIG. 14 which illustrates a fourteenth embodiment, the optical fiber cable 1 is comprised of two notches 51 formed on an outer surface of the sheath 9, which run along the central axis C. The notches 51 facilitate splitting of the sheath 9 at the time of the mid-span access process. The number of the notches 51 is not limited to two but may be three or more. Further, the sections of the notches 51 are not limited to a V-letter shape but may be any proper shape. The other aspects of the current embodiment may be identical to those of the first embodiment or the other.

The notches 51 (or, outermost two of the notches 51 in a case where three or more notches are provided) may be disposed symmetrically with respect to the thinnest wall portion 15 and leave a properly narrow interval therebetween. Thus positioned notches 51 are located at relatively thin portions of the sheath 9 and relatively close to each other, thereby sufficiently facilitating splitting of the sheath 9. A part of the sheath 9 separated from the remaining sheath 9 may be beneficially reused for the purpose of covering the exposed slot 11 as will be described later.

The breadth between the notches 51 affects easiness of re-enclosure of the optical fibers and easiness of ripping of the sheath. The test results summarized in Table 3 demonstrate this feature. In these tests, test pieces 1-6 respectively having various breadth were tested. In Table 3, breadths are indicated by angles alpha, where an angle alpha is defined as an angle NCN′ formed by planes CN, CN′ emanating from the central axis C and respectively passing the notches 51.

TABLE 3 TEST TEST TEST TEST TEST TEST PIECE 1 PIECE 2 PIECE 3 PIECE 4 PIECE 5 PIECE 6 CONDITION BREADTH 15° 20° 30° 90° 160° 180° BETWEEN NOTCHES (ANGLE α) RESULTS EASINESS OF DIFFICULT NOT SO EASY EASY EASY EASY DIFFICULT RE-ENCLOSURE DIFFICULT TO REQUIRE EASY TO THE FIBERS OF OPTICAL COVER STRESS TO RE-ENCLOSE ARE LIKELY FIBERS THE C-GROUP OPEN AND THE FIBERS TO RUN OFF CORES WITH CLOSE THE ONCE THE SHEATH SEPARATED SHEATH EASINESS OF DIFFICULT DIFFICULT EASY EASY EASY EASY RIPPING OF UNEXPECTED UNEXPECTED SPLIT AT THE THE SHEATH SPLIT MAY SPLIT MAY NOTCHES AS OCCUR OCCUR EXPECTED

As will be understood from Table 3, two or more notches on a sheath facilitate splitting of the sheath, much more do those having alpha-angles greater than 30 degrees (test pieces 3-6). Meanwhile, the test pieces 3-5 having alpha-angles in a range in a range from 30 degrees to 160 degrees are more advantageous in light of easiness of re-enclosure of optical fibers than the test pieces 1, 2 and 6 having alpha-angles out of the range.

The depth t of the notches 51 may be set in a particular range in relation to the thickness D of the sheath 9 at the part of the notches 51. Relatively small depths t in a range satisfying an inequality of t<D/5 are disadvantageous in light of easiness of splitting, and too large depths t in a range satisfying an inequality of t>4D/5 are disadvantageous in light of toughness of the sheath 9. Therefore the depth t in a range from D/5 to 4D/5 may be preferable.

Referring to FIGS. 15 and 16, a process of mid-span access will be described in relation to the optical fiber cable according to the fourth embodiment.

Referring to FIG. 15(A), a sharp edge of a cutter 25 is put into the sheath 9 and made advance around the circumference thereof to carry out cutting around of the sheath 9. Movement of a sharp end of the cutter 25 may be on a track illustrated as a round broken line in FIG. 16.

Next, as shown in FIG. 15(B) and FIG. 17, the sheath 9 is made split with the help of the notches 51. Then the sheath 9 is partly separated into a larger part and a smaller part, thereby the slotted core 7 is partly exposed in between these parts.

Referring to FIG. 15C, the smaller part of the sheath 9 is allowed to be cut out but the larger part is preferably left. One or more of the optical fibers 3 are pulled out of the slotted core 7 and then subject to a branching process. The rest of the optical fibers 3 are returned to and re-enclosed with the slotted core 7.

Then the uncut part of the sheath 9 can be reused for the purpose of covering the exposed slot 11 as shown in FIG. 15D. While the optical fiber(s) 3 branching off the optical fiber cable 1 is led out of the sheath 9, the remaining optical fibers 3 are re-enclosed with the sheath 9. This process does not require any specific jigs and can be executed by hands and fingers.

The fourteenth embodiment provides such reusability of the split sheath 9 as a cover of the exposed optical fibers 3, while such reuse may be made possible even in the other embodiments when some additional labors are added. In contrast, in the prior arts, an exposed core requires any tape wrapped around the core or such protection means so as to avoid damage to the optical fibers. Wrapping a tape on the core is considerably laborious. Therefore, the fourteenth embodiment provides both excellent protection and easy workability.

The aforementioned first through fourteenth embodiments are compatible with each other. Therefore, any combination of these embodiments will occur. Further, additional rip cords may be interposed between the slotted core 7 and the sheath 9.

Although the invention has been described above by reference to certain exemplary embodiments of the invention, the invention is not limited to the exemplary embodiments described above. Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings.

Industrial Applicability

Optical fiber cables enclosing fibers, in which enclosed fibers are easily accessible but prevented from damage, are provided. 

The invention claimed is:
 1. An optical fiber cable having an axis, the optical fiber comprising: a slotted core elongated along the axis, the slotted core including a slot running in parallel with the axis and a groove accessible through the slot; one or more optical fibers placed in the groove; a sheath enclosing the slotted core and the optical fibers; a bonding portion where the slotted core is bonded with the sheath; and two or more strength members embedded in the slotted core, the strength member running in parallel with the axis and being aligned on a plane including the axis.
 2. The optical fiber cable of claim 1, wherein the strength members include one selected from the group of steel and FRP.
 3. The optical fiber cable of claim 1, wherein the bonding portion includes one selected from the group consisting of a projecting rib projecting from the slotted core, a binding element interposed between the slotted core and the sheath, a recess receding in the slotted core, and a string having adhesive interposed between the slotted core and the sheath.
 4. The optical fiber cable of claim 1, further comprising: an absorptive yarn placed in the groove.
 5. The optical fiber cable of claim 1, further comprising: an elongate tape attached on the slotted core to cover the slot.
 6. The optical fiber cable of claim 5, wherein the elongate tape includes an absorptive tape.
 7. The optical fiber cable of claim 5, wherein the bonding portion is left uncovered by the elongate tape and aligned with the slot and the strength members on the plane.
 8. The optical fiber cable of claim 5, wherein the slot and the elongate tape are so dimensioned that a theta-angle formed by planes emanating from the axis and respectively in contact with edges of the slot of the slotted core is in a range from 30 degrees to 90 degrees, and a gamma-angle formed by another planes emanating from the axis and respectively in contact with both edges of the elongate tape is greater than the theta-angle and smaller than four times the theta-angle.
 9. The optical fiber cable of claim 1, further comprising: one or more anchors configured to support one or more of the optical fibers, the anchors being disposed at intervals in a direction along the axis.
 10. The optical fiber cable of claim 9, wherein each of the anchors includes a UV-setting resin having a Young's modulus of 800 MPa or less and a viscosity of 500 cps or more at a normal temperature, each of the intervals between the anchors is in a range from 100 mm to 2000 mm, and a force required to draw out the supported optical fiber is 5N/10 m or more.
 11. The optical fiber cable of claim 1, wherein the sheath includes a nonuniform wall so that a largest thickness of the wall is 1.5 times or more of a smallest thickness of the wall.
 12. The optical fiber cable of claim 1, further comprising: a marker formed on the sheath, the marker indicating a position of the bonding portion.
 13. The optical fiber cable of claim 1, further comprising: two or more notches formed on an outer surface of the sheath and running along the axis.
 14. The optical fiber cable of claim 13, wherein the notched are disposed so as to have an alpha-angle formed by planes emanating from the axis and respectively passing two of the notches in a range from 30 degrees to 160 degrees.
 15. A method of mid-span access of the optical fiber cable of claim 1, the method comprising: splitting the sheath in part to expose the slot in part and separate a part of the sheath; pulling one or more of the optical fibers out of the slotted core; and covering the exposed slot in part with the separated part of the sheath. 